1. Field of the Invention
The present invention generally relates to a DC-to-AC power converting apparatus. More particularly, the present invention is directed to a DC/AC power converting apparatus including either a DC power source, or a DC energy storage element; a DC/AC power inverting circuit; a control circuit for controlling either an output voltage, or an output current of the DC/AC inverter circuit; and also a DC component removing circuit.
2. Description of the Related Art
The DC/AC power converting apparatus set forth in the preamble is known in this field from, for instance, "A SEMICONDUCTOR POWER CONVERTING CIRCUIT" issued on Mar. 31, 1987 from Ohm-sha Co., Ltd. in Japan, FIG. 6.3.53 on page 150.
In FIG. 1, there is shown a conventional DC/AC power converting apparatus. A DC power supply 1 is connected to supply DC power to a DC/AC inverter circuit 2. The DC/AC inverter circuit 2 is arranged by, for instance, a switching transistor, IGBT (insulated-gate bipolar transistor), gate turn-off thyristor (GTO) and the like. AC power produced from this DC/AC inverter circuit 2 is supplied via a transformer 3 to a load, or other power supply systems.
An output current or output voltage of the DC/AC inverter circuit 2 is detected by a current detector 5 such as a current transformer, and supplied as a detected current signal "i.sub.D " to a calculation circuit 6. A reference current signal "i.sub.R " is also supplied to this calculation circuit 6. In the calculation circuit 6, the detected current signal "i.sub.D " is compared with the reference current signal "i.sub.R ", and a controlling calculation, e.g., a proportional integration is carried out in such a manner that a difference between the detected current signal "i.sub.D " and the reference current signal "i.sub.R " becomes a small value. As a result, an output instruction signal V.sub.I is produced from this calculation circuit 6 and then supplied to a gate drive circuit 8. The output instruction signal "V.sub.I " is converted into a gate signal "S.sub.g " in the gate divine circuit 8. Then, the gate signal "S.sub.g " is supplied to the DC/AC inverter circuit 2 so that the AC output from the DC/AC inverter circuit 2 follows the above-described reference current value "i.sub.R ". More specifically, the inverting operation of the DC/AC inverter circuit 2 is controlled based upon this gate signal "S.sub.g " in the PWM (pulse width modulation) form.
The above-described current detector 5, and calculation circuit 6 will constitute part of a current, or voltage feedback loop. If a DC component is produced due to a change in offset values in the feedback loop circuit, the AC output of the DC/AC inverter circuit 2 contains DC components. Since the output terminal of this DC/AC inverter circuit 2 is connected to the transformer 3 made of an iron core, the DC components contained in the AC output therefrom may cause DC magnetization of the transformer 3. For instance, the current transfer has the offset value. Moreover, even if the offset adjustment would be effected at the beginning, a drift may occur in the offset value, due to aging offset. As a consequence, a power loss in the transformer 3 is increased, and furthermore noises are produced therein. In the specific conditions, the transformer 3 is saturated so that no power can be supplied to the load 4, or an excessive current output from the DC/AC inverter circuit 2 flows through the output circuit thereof. Accordingly, the performance of the DC/AC inverter circuit 2 may be considerably deteriorated and the reliabilities of the overall DC/AC power converting apparatus may be lowered.